Freeze-drying is a method for removal of previously frozen water, for example from drugs and food. In general, the method is carried out at an air pressure which is low vis-a-vis the water vapor pressure at the selected temperature of the ice. For example, an ice temperature of -20.degree. C. corresponds to a water vapor pressure (in equilibrium) of 1.03 mbar. In order for the water vapor to be able to flow from the surface of the ice into the drying chamber, the water vapor pressure in the drying chamber must be distinctly lower than 1.03 mbar, i.e. for example 0.4 mbar. According to the preceding statement, the air pressure must be low vis-a-vis 0.4 mbar, i.e. for example 0.05 mbar.
As long as there crystallized (frozen) water remains in the product, this drying segment is called the primary- or the sublimation drying. If, during this phase of drying, a shut-off device is closed for a short time (seconds) between chamber and condenser, there occurs in the chamber the equilibrium water vapor pressure which corresponds to the prevailing ice temperature. From the increase in pressure it is possible to draw a direct conclusion with respect to the ice temperature. This method is called barometric temperature measuring (BTM).
When there is no longer any water in the form of ice present, the remaining water is absorbed by the dry product or is more or less solidly bonded thereto. Removal of this water is called secondary or desorption drying. The desorbable water volume during this phase depends upon the temperature of the product, the type of water bond and the still present water volume. In the thermo-dynamic equilibrium, the respective water vapor pressure (for a given temperature) constitutes a measure for the water content of the dry product. This function: water vapor pressure as a function of water content of the dry product is called desorption isotherm. The establishment of this equilibrium takes, however,--depending upon temperature--on the order of hours. During a drying process which, in general, lasts only a few hours, it is not possible to repeatedly determine this measuring quantity. It is therefore customary to remove from the drying chamber, during the secondary drying, single bottles containing dry product or a specimen from the bulk product with a vacuum manipulator, to determine the residual moisture (RM) and to terminate the drying process upon reaching the desired RM. If such device is not available, it is necessary to run several batches, to select different drying times and to measure the proper RM. In that case one has to rely, however, on that the process cycle is always identically controlled. Certain deviations are hereby unavoidable, and it is always a risk to estimate which deviations are still tolerable and which are not.
In addition, both methods have the serious drawback that the RM can only be determined on the basis of several samples. As experience demonstrates, the RM values of several samples can substantially fluctuate, depending, for example, upon the location of the bottles or the location of the bulk product in the chamber. Therefore it is desirable to measure an average RM for all bottles (which may amount to a total of up to several thousands) or across the entire bulk product (which may amount up to several tons).